Abstract: Under the strategic goal of “3060 double carbon”,new energy and renewable energy are deeply building a clean energy system. Aiming at the optimization problem that energy storage provides frequency regulation auxiliary services to the power grid as an independent subject to deal with intermittent and random power fluctuations,the capacity allocation strategy of hybrid energy storage system（HESS）of flywheel and lithium battery is proposed. Firstly,the mathematical model is established to maximize the economic benefits of energy storage considering frequency regulation mileage. Then the command is decomposed into high frequency demand and low frequency demand by using complete ensemble empirical mode decomposition with adaptive noise（CEEMDAN）. Finally,embedded with the charge and discharge power control and state of charge（SOC）constraints of hybrid energy storage,an example is constructed under different frequency division parameters and solved by particle swarm optimization that is improved by opposite search strategy in the early stage and mutation and crossover strategy in the later stage. The results show that the configuration scheme obtained by this method effectively improves the overall economy and safety of hybrid energy storage.